In mobile communications systems, signal performance may be reduced due to signal fading occurring as a result of physical interference and/or motion of the mobile terminal. Fading can be reduced by a number of methods, such as increasing transmitter power and/or antenna size and height. However, a more cost effective method is to implement transmit diversity at the base station, in systems such as wideband code division multiple access (“WCDMA”) and CDMA-2000, in order to improve link quality. Transmit diversity provides an advantage in the forward link by balancing the spectrum efficiency in the uplink and downlink.
Several transmit diversity techniques are known in the art. Among these are orthogonal transmit diversity (“OTD”) and time-switched transmit diversity (“TSTD”). OTD is a technique in which a data stream is split into two (2) separate streams that are then transmitted on two (2) transmit antennae at the same time by employing different Walsh codes. TSTD is a technique in which symbols of a data stream are alternately transmitted from two (2) transmit antennae. One problem with OTD and TSTD is that full diversity is not achieved if the interleaver is not well-designed. For example, if a data stream is convolutionally encoded and many consecutive bits are transmitted from the same antenna, then there is not much gain over simply using a single transmit antenna.
It is therefore desirable to provide a solution that adequately exploits the benefits obtainable from using transmit diversity. The present invention provides this by designing the size of the interleaver matrix to avoid the case where most or all of the bits in a row are transmitted on a single antenna. This can be accomplished, for example, by specifying the interleaver matrix such that the number of rows is an odd multiple of the number of bits/symbol or such that the number of rows is determined by the type of modulation.